Molecular Design for Long-Range Electronic Communication between Metals

Abstract

To construct molecular devices it is necessary to use mixed-valence metal complexes which have a large metal-metal separation distance and which exhibit strong coupling between the metals, so that errors which might arise from electrostatic interaction between the metal ions are prevented. Bridges, or spacers, are needed between two metal terminal sites to operate as effective molecular wires when one metal terminal site is in the excited state, and/or when both the terminal components are in the ground state. Binuclear ruthenium complexes, consisting of tris(β-diketonato)ruthenium(III) units, which are suitable as the terminal redox sites, can be used to evaluate how well the bridges function as molecular wires in the ground state. This is because their Ru(III)-Ru(II) and Ru(IV)-Ru(III) mixed-valence states are accessible for experimental use. In this article, a polyyne system and an ethynylated aromatic system are evaluated as molecular wires, using the binuclear (β-diketonato)ruthenium(III) complexes containing these systems as the bridges. In the Ru(IV)-Ru(III) mixed-valence state, the ruthenium complexes show relatively strong electronic interactions between the metal centres. This is interpreted by a superexchange (through-bond) hole transfer mechanism via the highest occupied molecular orbitals of the bridge. Molecular orbital calculations provide a guide to the molecular design of bridging ligands for long-range electronic coupling.